Manganese is transported into brain by olfactory neurons. Manganese and manganese oxide particles are known to accumulate in brain tissue in welders and in manganese miners resulting in neurotoxic manifestations. Homeostatic mechanisms keep blood manganese levels tightly controlled, but over-exposure can overload the excretion process resulting in toxic accumulation of the metal in the globus pallidus and other brain regions. Welders and miners with over-exposure have had MRI images taken that reveal increased signal in the globus pallidus (structure of the basal ganglia) under T-1 weighted conditions. However, clearance of manganese and reversal of the image to normal occurs when subjects are taken out of the toxic environment, emphasizing powerful homeostatic mechanisms to keep manganese tissue levels within a narrow range.
Manganese, like gadolinium, is useful as a contrast agent. Infusion of manganese oxide into the venous system will permit visualization in the brain of leaky blood vessels associated with various disease processes such as stroke, inflammation, trauma.
The mechanism of transport of manganese into the brain via the olfactory system, when studied in rat, revealed that manganese transport was saturable when considering uptake into olfactory bulb (Henriksson et al., (1999) Toxicol. App. Phamacol. 156: 119-128). In addition, manganese was reported to move relatively freely from the olfactory bulb to olfactory cortex and other regions of brain. Thus, olfactory neurons provide a pathway with considerable capacity to transport manganese into the brain.
Nanoparticles can be designed to deliver genes of interest to specific targets such as tumors. Delivery of therapeutic genes into the central nervous system has mostly relied on stereotaxic neurosurgical injection into the brain of gene products (e.g. growth factors) or viral vectors that can include a gene of interest desired to be expressed in the brain. For chronic diseases like Huntington 's disease that afflict the entire brain over extended periods, injection of a vector into multiple brain regions for a prolonged time make it unfeasible and unacceptable. Another problem with the current state of the art is the reliance on viral vectors (adeno-associated viruses or lentiviruses) to deliver a nucleic acid of interest. Applying these vectors to humans carries a degree of risk that is often unacceptable and can be obviated by using other vehicles for gene delivery such as nanoparticles designed to target specific tissues.
Gene delivery technology has grown very rapidly with applications designed to replace defective genes, substitute missing genes, or silence undesirable gene expression. However, naked genes are rapidly degraded by nucleases, showing poor cellular uptake, low target specificity, and inadequate transfection efficiency (Kim et al., (2007) Prop. Polym. Sci. 32: 726-753). Therefore, the development of efficient gene carriers is one of the prerequisites for the realization of gene therapy (Rolland A. (2005) Adv. Drug Deliv. Rev. 57: 669-673). Recently, chitosan-based carriers have become one of the major non-viral vectors that have received increasing interest as a reliable gene or siRNA delivery system. Chitosan has low toxicity, low immunogenicity, excellent biocompatibility (Shu & Zhu (2002) Eur. J. Pharm. Biopharm. 54: 235-243; Lee et al., (2005) Biomaterials 26: 2147-2156). Due to its positive charge, it can easily form polyelectrolyte complexes with negatively charged nucleotides by electrostatic interaction. However, the efficiency of chitosan to deliver gene therapy is significantly influenced by formulation.
Chitosan is obtained by deacetylation of chitin, which is the biodegradable polysaccharide consisting of repeating D-glucosamine and N-acetyl-D-glucosamine units, linked via (1-4) glycosidic bonds. Chitosan is almost non-toxic in animals (Rao & Sharma (1997) Biomed. Mater. Res. 34: 21-28) and humans (Aspden et al., J. Pharm. Sci. 86 (1997) 509-513), with an LD50 in rats of 16 g/kg (Chandy & Sharma (1990) Biomater Artif Cells Artif Organs 18: 1-24). Chitosan can be characterized by several physicochemical properties, including molecular weight, degree of deacetylation, viscosity, and crystallinity (Kas H. S. (1997) J. Microencapsul. 14: 689-711). The desirability of chitosan as a gene delivery carrier is based on its cationic property to allow binding of negatively charged siRNA via electrostatic interactions.